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Comm Antenna Radiation Pattern

 
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dreel(at)cox.net
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PostPosted: Sat Apr 01, 2006 3:36 am    Post subject: Comm Antenna Radiation Pattern Reply with quote

On my second test flight yesterday, The control tower's transmission during my initial climb was so weak I could just barely tell someone had transmitted. I was climbing with the tower at my back and at a pretty steep angle due to other problems. Except during this climb phase, I heard the tower loud and clear. Refering to the radiation pattern in figure 13-7 of the AeroElectric Connection book, I'm theorizing that my comm antenna, which is mounted beneath the left wing/fuselage joint so that about a half foot of the bent antenna protrudes behind the left flap on my RV8A, was blocked by the wing/fuselage from radiating towards the tower. So, I have two questions.

First, is the null on the ground plane side of this type of installation deep enough to attenuate the tower transmission so thoroughly?

Second, would moving the antenna further aft reduce this problem?

Dave Reel - RV8A


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brian



Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

PostPosted: Sat Apr 01, 2006 8:24 am    Post subject: Comm Antenna Radiation Pattern Reply with quote

DAVID REEL wrote:
Quote:


On my second test flight yesterday, The control tower's transmission during my initial climb was so weak I could just barely tell someone had transmitted.
> ...

Quote:
First, is the null on the ground plane side of this type of installation deep enough to attenuate the tower transmission so thoroughly?

It could be. There really is no way to tell without testing with
something like a far-field field-strength meter. You should do some
flyover tests with someone on the ground who has a receiver with a
signal strength meter so you can plot signal strength vs. position. It
is tough to do on the ground because of ground proximity distorting the
radiation pattern.

Quote:
Second, would moving the antenna further aft reduce this problem?

Any time you can put the antenna on an uncluttered ground plane with
clear line-of-sight is going to be better but there are so many
variables in this picture that I just can't advise you.

Again, if you can mount it on an uncluttered ground plane with a clear
LoS it will work as good as it can. The gear legs very well could be
distorting the radiation pattern.

--
Brian Lloyd 361 Catterline Way
brian-yak at lloyd dot com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery


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PostPosted: Sat Apr 01, 2006 4:11 pm    Post subject: Comm Antenna Radiation Pattern Reply with quote

At 06:30 AM 4/1/2006 -0500, you wrote:

Quote:


On my second test flight yesterday, The control tower's transmission
during my initial climb was so weak I could just barely tell someone had
transmitted. I was climbing with the tower at my back and at a pretty
steep angle due to other problems. Except during this climb phase, I
heard the tower loud and clear. Refering to the radiation pattern in
figure 13-7 of the AeroElectric Connection book, I'm theorizing that my
comm antenna, which is mounted beneath the left wing/fuselage joint so
that about a half foot of the bent antenna protrudes behind the left flap
on my RV8A, was blocked by the wing/fuselage from radiating towards the
tower. So, I have two questions.

First, is the null on the ground plane side of this type of installation
deep enough to attenuate the tower transmission so thoroughly?

Second, would moving the antenna further aft reduce this problem?

The variables here are huge. First, we know that the radiation/reception
pattern of antennas mounted on aircraft seldom have lab-perfect
shapes. Further, depending on operating frequency, antenna location
and aircraft geometry, the variability from minimum to maximum
performance can be large . . . notice "large" is not quantified.

If you just departed an airport and had trouble communicating from
less than 5 miles away, I'm skeptical that ordinary distortions
of the antenna's pattern could cause the tower to drop away markedly
in strength. They use 20-50 watt transmitters and very omni-directional
antennas.

You'll need to conduct some experiments to see if you can duplicate the
condition. Then vary your heading by 20 degrees or so either side of
the "null" without changing deck angle. If the signal strength comes
up markedly by changing directions, then pattern is certainly a component
of your difficulty. How's your performance at altitude for stations out
at the horizon?

One of the problems for testing comm antenna patterns is that there
are no steady-on ground based signal sources like we have with
VOR, LOC, VOT, etc.

At RAC, we have a steady-on beacon we can use to radiate for flight
testing antennas. The aircraft flies a zero-bank 360 out 100 miles
or so and we plot the received signal strength from the ground
beacon. Gives very nice pattern data.

You may be suffering from both pattern effects and efficiency problems.
A weakly performing antenna will display pattern effects that are
otherwise quite tolerable. I've had a friend stand in a clear space
with a hand held and give me several minutes of long "aaaaaahhhhhhh" while
I did the a merry-go-'round maneuver to check for deep nulls.

Be sure the hand-held will tolerate this. MANY comm transmitters are not
rated for long transmissions like this. If the hand-held has a high-low
power setting, you can certainly do it in the low power setting.

Bob . . .


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